Add the ability to change a vhd's parent (locators)

This is useful for reconstructing chains from individual pieces.

Signed-off-by: David Scott <dave.scott@citrix.com>
Add a unit test for the parent changing functionality

Signed-off-by: David Scott <dave.scott@citrix.com>
Update CHANGES for a 0.7.2

Signed-off-by: David Scott <dave.scott@citrix.com>